Canter on a figure of eight, when you go to change reign bring her back to trot and pick up canter on the other lead, do this continuously doing the amount of trot in between smaller Sachs time.
I haven't gotten this far myself, but I've read a lot in magazines. They suggest that you first get confident in asking for canter and getting the correct response and lead. Then you add in a down transition to trot before asking for the opposite lead. Once this is perfect you can then reduce the amount of trot in between to just a few strides and down to 2 strides of trot and eventually cut out trot altogether.